However, today I found TF2LS, and I think it has the most concise and easy-to-read interface for seeing what items you are missing and what items you have too much. I believe this is the best site to answer your question.

I use tf2ls for the purpose, but I also just found this built into TF2:

Go to the Mann Co. Store.

Choose the “Weapons” tab.

From the “Show” menu, select “Unowned Items”.

Of course, this will not show weapons not available in the store. The only such weapons currently (October 14, 2011) which have unique characteristics are the Vita-Saw, Frying Pan, Sharpened Volcano Fragment, and Sun-on-a-Stick.